Since 14" wheels and tires will work fine over stock 13", as confirmed by me personally calling Scamp. That would give the 1" to 1 1/2" extra clearance to run a 1 1/2" pipe from the grey tank forward where it can be integrated into a 2 into 1 Y with seperate valves. Sounds easy enough.

Not even looking at it Frederick. I need to get over there and see if the axle is in the way and some other factors before I can give a definitive answer. I did however give it a cursory look while on the road this past summer and it looked very doable to me. As far as slope all of the grey water tank will be above the drain and it didn't appear, on first glance, to have any slope. But, even if it did and only drained the top half that would be livable.

Hummm. The size of the tires don't have anything to do with it. It's the axle placement. I have 15" tires on my Scamp.

Unless you have a spacer between the axle and the frame... the axle is right against the cross member on the frame. There isn't any space between the floor, the frame and the axle to run a pipe. You'll need to run the pipe under the axle. If you only drain half the gray water, that will leave 13 gallons in the tank.